DEFINITION OP THE TERM INSECT. 45 



in which the vertebral column becomes external or merges 

 in the upper shell. The cyclostomous fishes also are not 

 very wide of insects as to their integument. But on this 

 subject I shall be more full hereafter. 



The forms of insects are so infinitely diversified that 

 they almost distance our powers of conception : in this re- 

 spect they seem to exceed the fishes and other inhabitants 

 of the ocean, so that endless diversity may be regarded 

 as one of their distinctions. But on all their variations 

 of form the Creator has set his seal of symmetry ; so that, 

 if we meet with an animal in the lower orders in which 

 the parts are not symmetrical, we may conclude in general 

 that it is no insect. 



But it is by their parts and organs that insects may be 

 most readily distinguished. In the vertebrate animals, 

 the body is usually considered as divided into head, 

 trunk, and limbs, the abdomen forming no part of the 

 skeleton ; but in the insect tribes, besides the organs of 

 sense and motion, the body consists of three principal 

 parts — Head, Trunk, and Abdomen — the first, as was 

 before observed, bearing the principal organs of sense 

 and manducation ; the second most commonly those of 

 motion ; and the third those of generation — the organs of 

 respiration being usually common to both trunk and ab- 

 domen. These three primary parts, — though in some in- 

 sects the head is not separated from the trunk by any 

 suture, as for instance in the Arachnid a ,• and in others, 

 head, trunk, and abdomen form only one piece, as in some 

 mites, — still exist in all, and in the great majority they are 

 separated by incisures more or less deeply marked : this 

 is particularly visible in the Hymenoptera and Diptera, 

 which, in this respect, are formed upon a common model ; 



